Question

A convex lens of focal length $40\, cm$ is in contact with a concave lens of focal length $25 \,cm$. The power of the combination, is

• A. + 6.5 D
• B. - 6.5 D
• C. - 1.5 D
• D. + 6.67 D

Answer 1

Answer: (C)

- 1.5 D

Answer 2

Given: Focal length of a convex lens $\left(f_{1}\right)=40\, cm =0.4\, m$ and focal length of a concave lens $\left(f_{2}\right)=-25\, cm =-0.25 \,m$ (minus sign due to concave lens). We know that relation for the focal length of the combination of lenscs $\frac{1}{f}=\frac{1}{f_{1}}+\frac{1}{f_{2}}$ $=\frac{1}{0 \cdot 4}+\frac{1}{-0 \cdot 25}$ $=2.5-4=-1 .5\, D$ or power $(P)=\frac{1}{f}=-1.5\, D$